Photophysical properties of dioxolane-substituted pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III)

Two novel dioxolane-substituted pentacene derivatives, namely, 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (TP-5) and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5), have been synthesized and spectroscopically characterized. Here, we examine the steady-state and time-resolved photoluminescence (PL) of solid-state composite films containing these pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq(3)). The films show narrow red emission and high absolute photoluminescence quantum yields (phi(PL) = 59% and 76% for films containing approximately 0.25 mol % TP-5 and EtTP-5, respectively). The F?rster transfer radius for both guest-host systems is estimated to be approximately 33 A. The TP-5/Alq(3) thin films show a marked decrease in phi(PL) with increasing guest molecule concentrations, accompanied by dramatic changes in the PL spectra, suggesting that intermolecular interactions between pentacene molecules result in the formation of weakly radiative aggregates. In contrast, a lesser degree of fluorescence quenching is observed for EtTP-5/Alq(3) films. The measured fluorescence lifetimes of TP-5 and EtTP-5 are similar (approximately 18 ns) at low concentrations but deviate at higher concentrations as aggregation begins to play a role in the TP-5/Alq(3) films. The onset of aggregation in EtTP-5/Alq(3) films occurs at higher guest molecule concentrations (>1.00 mol %). The addition of ethyl groups on the terminal dioxolane rings leads to an increase in the intermolecular spacing in the solid, thereby reducing the tendency for pi-pi molecular stacking and aggregation.     

Dynamics of energy transfer of a dioxolane-substituted pentacene dispersed in 4,4-bis[N-1-naphthyl-N-phenylamino]biphenyl

We examine the steady-state and time-resolved photoluminescence of guest-host films featuring a dioxolane-substituted pentacene derivative (2,2,10,10-tetraethyl-6,14-bis(triisopropylsilylethynyl)-1,3,9,11-tetraoxadicyclopenta[b,m]pentacene, EtTP-5) dispersed in the hole transporting material (4,4-bis[N-1-naphthyl-N-phenylamino]biphenyl, alpha-NPD). The films show bright red emission (lambda(max) = 640 nm) as a result of efficient F?rster energy transfer from alpha-NPD host molecules to EtTP-5 guest molecules. High absolute photoluminescence (PL) quantum yield (phi(PL) = 76% +/- 4%) and fluorescence lifetime (tau = 18.6 +/- 0.8 ns) were measured at low concentration (0.28 mol % EtTP-5), with moderate PL quenching observed upon increasing the EtTP-5 concentration. The concentrated films (> or = 1.50 mol % EtTP-5) show less evidence of aggregation than previously seen when EtTP-5 was dispersed in tris(quinolin-8-olato)aluminum(III), making alpha-NPD a superior host for the red-emitting EtTP-5.     

Effect of electronic polarization on charge-transport parameters in molecular organic semiconductors

Theoretical investigations of charge transport in organic materials are generally based on the "energy splitting in dimer" method and routinely assume that the transport parameters (site energies and transfer integrals) determined from monomer and dimer calculations can be reliably used to describe extended systems. Here, we demonstrate that this transferability can fail even in molecular crystals with weak van der Waals intermolecular interactions, due to the substantial (but often ignored) impact of polarization effects, particularly on the site energies. We show that the neglect of electronic polarization leads to qualitatively incorrect values and trends for the transfer integrals computed with the energy splitting method, even in simple prototypes such as ethylene or pentacene dimers. The polarization effect in these systems is largely electrostatic in nature and can change dramatically upon transition from a dimer to an extended system. For example, the difference in site energy for a prototypical "face-to-edge" one-dimensional stack of pentacene molecules is calculated to be 30% greater than that in the "face-to-edge" dimer, whereas the site energy difference in the pentacene crystal is vanishingly small. Importantly, when computed directly in the framework of localized monomer orbitals, the transfer integral values for dimer and extended systems are very similar.     

Thermally induced solid-state phase transition of bis(triisopropylsilylethynyl) pentacene crystals

Bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) is a functionalized pentacene derivative designed to enhance both the solution solubility and solid-state packing of pentacene. In this paper, we report our observations of a solid-state phase transition in TIPS pentacene crystals upon heating or cooling. Evidence from differential scanning calorimetry (DSC), hot-stage optical microscopy, as well as high-temperature X-ray and electron diffraction are presented. A reasonable match with experimental data is obtained with molecular modeling. Our results reveal that the transition is associated with a conformational reorganization of the TIPS side groups, accompanied by a slight decrease in the acene-to-acene spacing and a shift of the overlap between the neighboring pentacene units. The observed cracking should be avoided or minimized in TIPS pentacene-based thin film transistors to maintain their relatively high charge carrier mobility.     

Substituent effects on packing entropy and film morphologies in the nucleation of functionalized pentacenes on SiO(2) substrate: Molecular dynamics simulations

The changes in structural ordering, packing entropy, free energy, and film morphologies in the initial nucleation processes of pentacene (Pn), 6, 13-bis(t-butylethynyl) pentacene (t-Bu Pn), and 6, 13-bis(triisopropylsilylethynyl) pentacene (TIPS Pn) on the SiO(2) substrate were investigated, by using the molecular dynamics simulations. During the nucleation, the rod-like Pn molecules tend to diffuse rapidly and have different orientations on the SiO(2) surface. At the low coverage, the t-Bu Pn and TIPS Pn molecules with the branched topological structures almost lie flat on the substrate. On the basis of statistical distribution of various packing configurations of the functionalized Pn pairs, the packing entropy is estimated according to the Boltzmann formula. The packing entropy abruptly decreases in the early stage of deposition. Once the critical nucleus size is reached, the packing entropy converges to a constant value. As the coverage increases, the monolayer films of Pn and its branched derivatives become more ordered. The TIPS Pn with the relatively larger molecular area would occasionally stand on the surface during the nucleation, resulting in the dramatic changes in free energy. In the monolayers, the functionalized Pn molecules are packing more orderly than those in amorphous solids, but less orderly than those in crystals. The degree of order of these monolayers increases as the size of the substituents increases. The understanding of substituent effects on nucleation processes and packing structures is helpful to fabricate organic thin films with well-predefined molecular orientations.     

Thermal and mechanical cracking in bis(triisopropylsilylethnyl) pentacene thin films

Bis(triisopropylsilylethnyl) pentacene (TIPS pentacene) was synthesized to increase its solubility in common liquid solvents and, at the same time, enhance the π–π stacking between neighboring acenes in the crystallized state in comparison with unmodified pentacene. Hot-stage microscopy experiments revealed that during heating voids develop along the long axis of the TIPS pentacene films {along the [210] direction/parallel to the (120 ) planes} and crystals overlap along the short axis {along the [120 ] direction/parallel to the (210) planes}. From molecular mechanics simulations, the predominant twin boundaries of (120 ) and commonly observed cracking planes of (120), (120 ), and (210) had relatively low surface energies in comparison with planes with similar Miller indices. Organic thin-film transistors with TIPS pentacene as the active layer were fabricated, and the mobility values decreased from 0.4–1.0 cm²/V s before cracking to ∼0.2 cm²/V s after cracking. To maintain the high charge carrier mobility of TIPS pentacene devices, these cracks should be avoided. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3631–3641, 2006     

Density functional theory study on electron and hole transport properties of organic pentacene derivatives with electron-withdrawing substituent

Attaching electron-withdrawing substituent to organic conjugated molecules is considered as an effective method to produce n-type and ambipolar transport materials. In this work, we use density functional theory calculations to investigate the electron and hole transport properties of pentacene (PENT) derivatives after substituent and simulate the angular resolution anisotropic mobility for both electron and hole transport. Our results show that adding electron-withdrawing substituents can lower the energy level of lowest unoccupied molecular orbital (LUMO) and increase electron affinity, which are beneficial to the electron injection and ambient stability of the material. Also the LUMO electronic couplings for electron transport in these pentacene derivatives can achieve up to a hundred meV which promises good electron transport mobility, although adding electron-withdrawing groups will introduce the increase of electron transfer reorganization energy. The final results of our angular resolution anisotropic mobility simulations show that the electron mobility of these pentacene derivatives can get to several cm(2) V(-1) s(-1), but it is important to control the orientation of the organic material relative to the device channel to obtain the highest electron mobility. Our investigation provide detailed information to assist in the design of n-type and ambipolar organic electronic materials with high mobility performance.     

Methodologies for the synthesis of pentacene and its derivatives

In recent years, due to its high hole-mobility, high on/off current ratio and low threshold voltage, pentacene and its derivatives have found increasing application in the fabrication of light-emitting diodes, field-effect transistors and photovoltaic cells. It has also emerged as an alternative to silicon due to its similar performance to inorganic semiconductors. Pentacene cannot be isolated from the petroleum fractions like other acenes such as anthracene or tetracene, and therefore it needs to be chemically synthesized. The first successful synthesis of pentacene was reported in early 19th century where pentacene was obtained via dehydrogenation of 6,14-dihydropentacene. Since then a number of methods have been reported for the synthesis of pentacene. This review describes various strategies used for the synthesis of pentacene and its derivatives reported since 2005.     

The electronic structures and spectra of conducting pentacene derivatives

Theoretical studies of pentacene and a series of its derivatives were performed using the PM3 and DFT methods. Based on B3LYP/6-31G(d) optimized geometries, the electronic, IR, and ¹³C NMR spectra of the derivatives were calculated using the INDO/CIS, PM3, and B3LYP/6-31G(d) methods, respectively. The energy gaps of the derivatives decreased as the chain length increased and electron-withdrawing substituents were introduced. The polymer based on pentacene, especially in the presence of trimethylsilylacetylene, offers promise as an excellent conducting polymer. The main absorption bands in the electronic spectra of the derivatives compared with those of pentacene were shifted to the red, whereas the IR frequencies for some of the C=C and C-H bonds were shifted to the blue. The ¹³C chemical shifts of the carbon atoms connected with electron-withdrawing substituents were shifted upfield, while those of the bridged carbon atoms in the middle part of the pentacene unit shifted downfield.     

Cyanation: providing a three-in-one advantage for the design of n-type organic field-effect transistors

The theoretical work presented here demonstrates that, when substitution takes place at appropriate positions, cyanation could be a useful tool for reducing the internal reorganization energy of molecules. A molecular-orbital-based explanation is given for this fundamentally important phenomenon. Some of the cyanated pentacene derivatives (nCN-PENT-n) not only have internal reorganization energies for electron transfer (lambda(-)) smaller than that of pentacene, but the lambda(-) values are even of the same magnitude as the internal reorganization energy for hole transfer (lambda(+)) of pentacene, a small value that few organic compounds have surpassed. In addition, cyanation raises the electron affinity of the parent compound and may afford good electronic couplings between neighboring molecules, because of its ability in promoting pi-stacking. For the design of high performance n-Type Organic field-effect transistors, high electron affinities, large intermolecular electronic couplings, and small reorganization energies are necessary. Cyanation may help in all three aspects. Two cyanated trialkylsilylethynyl pentacene derivatives with known pi-stacking structures are predicted to provide reasonably small internal reorganization energies, large electronic couplings, and high electron affinities. They have the potential to outperform N-fluoroalkylated dicyanoperylene-3,4:9,10-bis(dicarboximides) (PDI-FCN(2)) in terms of electron mobility.     

Orientation of pentacene molecules on SiO2: from a monolayer to the bulk

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to study the orientation of pentacene molecules within thin films on SiO2 for thicknesses ranging from monolayers to the bulk (150 nm). The spectra exhibit a strong polarization dependence of the pi* orbitals for all films, which indicates that the pentacene molecules are highly oriented. At all film thicknesses the orientation varies with the rate at which pentacene molecules are deposited, with faster rates favoring a thin film phase with different tilt angles and slower rates leading to a more bulklike orientation. Our NEXAFS results extend previous structural observations to the monolayer regime and to lower deposition rates. The NEXAFS results match crystallographic data if a finite distribution of the molecular orientations is included. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent pi* orbitals, indicating a breakup of the pentacene molecule.     

Singlet and triplet transitions in UV absorption spectra of pentacene

The UV absorption spectrum of pentacene in hexane solution and the diffuse UV reflectance spectrum for its solid sample have been obtained. A spectral band due to structural features resulting from intermolecular interactions in the solid state was detected for the solid sample at the energy of 1.51 eV. The types of detected electronically excited states for pentacene have been determined, and dominating electronic configurations have been interpreted using TD B3LYP/6-31G quantum-chemical calculations. The types of pentacene occupied and unoccupied molecular orbitals involved in electronic transitions have been determined from published photoelectron spectroscopy data for pentacene with the preliminary assignment of the relevant photoionization bands using the B3LYP/6-31G calculation method.     

Stabilizing Pentacene By Cyclopentannulation

A new class of stabilized pentacene derivatives with externally fused five‐membered rings are prepared by means of a key palladium‐catalyzed cyclopentannulation step. The target compounds are synthesized by chemical manipulation of a partially saturated 6,13‐dibromopentacene precursor that can be fully aromatized in a final step through a DDQ‐mediated dehydrogenation reaction (DDQ=2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone). The new 1,2,8,9‐tetraaryldicyclopenta[fg,qr]pentacene derivatives have narrow energy gaps of circa 1.2 eV and behave as strong electron acceptors with lowest unoccupied molecular orbital energies between ?3.81 and ?3.90 eV. Photodegradation studies reveal the new compounds are more photostable than 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene).     

Synthesis and electronic properties of conjugated pentacene dimers

Conjugated pentacene dimers 1-3 were synthesized in two steps from readily available precursors. Noteworthy is the initial step, which assembles five independent fragments to form the carbon-rich molecular framework. Solution-cast films of these materials are air stable. Photocurrent measurements for solution-deposited thin films show that dimer 3 exhibits photoconductive gain >10.     

Imaging of crystal morphology and molecular simulations of surface energies in pentacene thin films

We have investigated the crystal growth of the organic semiconductor pentacene by complementing molecular simulations of surface energies with experimental images of pentacene films. Pentacene thin films having variations in thickness and grain size were produced by vacuum sublimation. Large (approximately 20 microm) faceted crystals grew on top of the underlying polycrystalline thin film. The films were characterized using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Single crystals most commonly grew in a truncated diamond shape with the largest crystal face, (001), growing parallel to the substrate. Crystal morphologies and surface energies were calculated using force field-based molecular simulations. The (001) surface was found to have the lowest energy, at 76 mJ/m(2), which was consistent with experimental observations of crystal face size. It was demonstrated that the morphology of the large faceted crystals approached the equilibrium growth shape of pentacene. From contact angle measurements, the critical surface tension of textured pentacene thin films in air was determined to be 34 mJ/m(2).     

Molecular orientation and film morphology of pentacene on native silicon oxide surface

The growth morphology and mechanism of pentacene films on native Si oxide surface have been studied by using high-resolution electron energy loss spectroscopy (HREELS), X-ray diffraction (XRD), and atomic force microscopy (AFM). Despite the good agreement between our own and the reported XRD results, the previous XRD interpretation that the pentacene molecules are tilt-standing on the substrate cannot explain our HREELS data. The HREELS results show that a substantial portion of the first two layers of pentacene molecules are tilted-standing or randomly oriented, whereas the upper-layer molecules are mostly lying flat to the substrate. AFM reveals that the first two layers of molecules form a flat and smooth surface, but the upper layers show a rough terrace structure with a mean-square roughness equal to the average thickness (without counting the first two layers). This relationship is explained by a theoretical model which assumes the pentacene molecules to remain on a particular molecule layer after arrival. The observed film growth morphology may have significant implication on the performance of electronic devices based on pentacene thin films. A plausible explanation was proposed for the discrepancy between the HREELS-indicated and the XRD-derived molecular orientations.     

Effect of molecule-substrate interaction on thin-film structures and molecular orientation of pentacene on silver and gold

Evaporated pentacene thin films with thicknesses from several nm to 150 nm on gold and silver substrates have been studied by ultraviolet photoelectron spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). It was found that pentacene thin-film structures, particularly their molecular orientations, are strongly influenced by the metal substrates. UPS measurements revealed a distinct change in the valence band structures of pentacene on Au compared to those on Ag, which is attributed to the different packing between adjacent molecules. Using NEXAFS, we observed 74+/-5 degrees and 46+/-5 degrees molecular tilt angles on Ag and Au, respectively, for all measured thicknesses. We propose that pentacene molecules stand up on the surface and form the "thin-film phase" structure on Ag. On Au, pentacene films grow in domains with molecules either lying flat or standing up on the substrate. Such a mixture of two crystalline phases leads to an average tilt angle of 46 degrees for the whole film and the change in valence band structures. STM and distance-voltage (z-V) spectroscopy studies confirm the existence of two crystalline phases on Au with different conducting properties. z-V spectra on the low conducting phase clearly indicate its nature as "thin-film phase".     

Modeling the polymorphism of pentacene

Thin films of pentacene are known to crystallize in at least four different polymorphs. All polymorphs are layered structures that are characterized by their interlayer spacing d(001). We develop a model that rationalizes the size of the interlayer spacing in terms of intralayer shifts of the pentacene molecules along their long molecular axes. It explains the wide variety of interlayer spacings, without distorting the herringbone pattern that is characteristic of many acenes. Using two simple theoretical models, we attempt to relate the intralayer shifts with the dominant, although weak, interatomic interactions (van der Waals, weak electrostatic, and covalent). For two polymorphs, a consistent picture is found. A full understanding of the other two, substrate-induced, polymorphs probably requires consideration of interlayer interactions.     

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.     

High‐Yield Excited Triplet States in Pentacene Self‐Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission

One of the major drawbacks of organic‐dye‐modified self‐assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13‐bis(triisopropylsilylethynyl)pentacene–alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time‐resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface.